The present invention relates to a technique for correcting image data and more particularly to a defective pixel correcting technique for correcting one or more of defective pixels present in visible light image data, where the pixels are determined as defective by using the visible light image data read from a photographic film based on a visible light and infrared light image data read from the film based on an infrared light.
A photographic film may have defects such as scratches, dust, dirt, etc., on its surface. There is a known technique available for correcting such defect by means of an image processing operation relying on e.g. a luminance adjustment technique or an interpolating technique when a photographically recorded image is read from such “defective” film to be printed on a print sheet or output on a display.
An example of such known technique corrects defects by adjusting luminance, taking advantage of the unique property of infrared light. That is, unlike the visible light, the infrared light, when irradiated on an exposed photographic film, is hardly affected by a photographic image recorded thereon, but affected only by such physical defect as scratches, dust or the like. More particularly, both infrared light and visible light are transmitted through an exposed photographic film, and the portion of the film where a pixel value of infrared image data is below a predetermined threshold value is determined as a defective portion. Then, to a pixel value of each color component (red (R), green (G), blue (B)) of this defective portion, a correction value is added, which corresponds to an amount of luminance attenuation occurring in the defective portion relative to a normal (non-defective) portion of the film, thereby to increase the luminance. In this way, the pixel value of each color component of the defective portion is adjusted or corrected relative to the normal portion by means of luminance enhancement (see e.g. U.S. Pat. No. 6,775,419).
However, this luminance adjustment technique is based on an assumption that the pixel values of the respective color components of the defective portion have experienced an equal amount of luminance attenuation. For this reason, if the amounts of attenuation in the pixel values of the respective color components differ from each other as may happen in the case of a scratch in an emulsion-applied surface or layer on the film, the above technique cannot correct this defect effectively.
In such case, an interpolation technique may be employed which corrects the defective portion by utilizing pixel values of non-defective, i.e. normal pixels adjacent thereto as reference values for the defect correction. However, if the defective portion includes a boundary (edge) or a pattern of the photographic image recorded on the film, such defective portion cannot be properly corrected by the above technique if it simply applies pixel values of adjacent non-defective pixels to the defective portion for its correction. In an attempt to cope with this problem, there is a known technique which detects a direction along which the image boundary is present and then effects the interpolation along that detected direction. More particularly, along a plurality of differing directions from the defective portion, the technique calculates image “attribute” values such as pixel value difference between normal pixels, a distance between the normal pixels, etc. Further, based on pixel values of the normal pixels present along the plurality of directions, an interpolation value is obtained for each direction. Then, a weighted average value for each direction is calculated by using the image attribute values as a weight, thereby to obtain a final correction value for use in the interpolation. (see for example, Japanese Patent Application No. 2001-78038 (pages 7-8, FIGS. 4-5 and FIG. 8)).
Therefore, in a conventional method for correcting a defective pixel present in visible light image data where pixels are determined as defective by using the visible light image data and infrared light image data, this is generally done by correcting defective pixels by using the luminance adjustment technique first and then effecting defective pixel correction by using the interpolation technique. In either the luminance adjustment technique or the interpolation technique, the correction is preceded by an automatic determination of a defective pixel based on a predetermined defect criterion. However, a certain defect actually present on a photographic film can be left undetected by such defect criterion. As a result, the defect can remain uncorrected at all or be corrected only in an unnatural way. For this reason, if needed, the automatically corrected image data is displayed on a monitor for the operator to locate any pixel which requires additional correction. This manual operation for locating pixels requiring additional correction demands significant skill and patience on the part of the operator.